Advent of Code. Year 2015. Day 14

Published: (3 min read)

--- Day 14: Reindeer Olympics ---

In this puzzle, the reindeer are racing each other and we have to work out who is going to win.

Table of Contents

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Part 1

In the first part we need to find out which reindeer has won and what distance it has traveled after exactly 2503 seconds.

First, I’ve defined a reindeer class with all the necessary properties. There we have things like speed, fly time and rest time. There is also a Points property, but we will need that in the second part.

private class Deer
{
  private readonly int _speed;
  private readonly int _flyTime;
  private readonly int _restTime;

  private int _flyTimeLeft;
  private int _restTimeLeft;
  private bool _isResting;

  public int TravelledDistance { get; set; }

  public int Points { get; set; }

  public Deer(int speed, int flyTime, int restTime)
  {
    _speed = speed;
    _flyTime = flyTime;
    _restTime = restTime;
    _flyTimeLeft = flyTime;
  }

  public void Tick()
  {
    if (_isResting)
    {
      _restTimeLeft--;
      if (_restTimeLeft != 0)
        return;

      _flyTimeLeft = _flyTime;
      _isResting = false;
      return;
    }

    _flyTimeLeft--;
    TravelledDistance += _speed;
    if(_flyTimeLeft != 0)
      return;

    _restTimeLeft = _restTime;
    _isResting = true;
  }
}

We also need a way to parse our input into an array of reindeer, something like this.

private static IEnumerable<Deer> GetDeers(IEnumerable<string> lines)
{
  return lines
    .Select(r => r.Split(" "))
    .Select(r => new Deer(int.Parse(r[3]), int.Parse(r[6]), int.Parse(r[13])));
}

The main thing that we need from the Deer class is a Tick method. It handles all the core logic. Basically, Tick() is called every second for each reindeer. And after 2503 seconds we can iterate over the array of reindeers and get the maximum TravelledDistance. And this will be our solution for the first part.

public object Part1(IEnumerable<string> lines)
{
  var deers = GetDeers(lines);
  var enumerable = deers as Deer[] ?? deers.ToArray();
  for (var i = 0; i < 2503; i++)
  {
    foreach (var deer in enumerable)
    {
      deer.Tick();
    }
  }
  return enumerable.Select(r => r.TravelledDistance).Max().ToString();
}

Part 2

In the second part, we need to change the way we calculate the winner. This time there is a point system (that’s why we needed the Points property in the Deer class) and we need to add points to the leading reindeer every second.

The solution is basically the same as in the first part, except that we also need to increase the Points property for the reindeer that’s currently winning.

public object Part2(IEnumerable<string> lines)
{
  var deers = GetDeers(lines);
  var enumerable = deers as Deer[] ?? deers.ToArray();
  for (var i = 0; i < 2503; i++)
  {
    foreach (var deer in enumerable)
    {
      deer.Tick();
    }

    var max = enumerable.Select(r => r.TravelledDistance).Max();
    enumerable = enumerable.Select(r =>
    {
      if (r.TravelledDistance == max)
        r.Points++;
      return r;
    }).ToArray();
  }

  return enumerable.Select(r => r.Points).Max().ToString();
}

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